Thermal protection systems (TPS) for space vehicles are seldom in the public eye. The general public typically hears only about heat shields and shuttle tiles, and then merely during a manned spacecraft mission. The fact is, thermal protection systems are essential for the successful launch and operation of all spacecraft, whether manned or unmanned.
The obvious goal of a thermal protection system is to keep excessive heat from destroying or damaging a vehicle or its contents. Thermal protection is not as important in vehicles that are designed to be expendable, or to function only for a short time. However, this is not generally the case in today's economy where costs require that vehicles and accessories be reused, even though most electronic equipment is susceptible to permanent damage when exposed to high temperatures. In fact, in order to meet mission objectives a TPS is subject to other constraints in addition to heat requirements. As an example the components must be protected with a minimal weight increase. As a whole, then, the TPS cannot be optimized from every point of view. Hence, neither a best thermal protection system, nor a universal criterion for rating the performance of a TPS is possible.
Without effective temperature thwarting, rockets of the future may never offer routine low-cost access to space. For today's space shuttle fleet, various heat protection approaches have been utilized. In addition to ceramic and metal coatings, various matrix materials have been tried on the various orbiters. They have been chosen for their weight efficiency, and constancy at high temperatures, which can approach 3,000 degrees Fahrenheit.
One early approach to thermal protection has been the use of ablators. Ablative materials, such as cork, Teflon, Lucite, fiberglass, nylon, and urethane, work by absorbing great amounts of heat through permanent phase changes. During their phase changes such ablators form an ash or char layer which acts as an insulator to protect the substrate underneath, but the substrate nevertheless continues to decompose and outgas. The gaseous products from decomposition percolate through the char to cool effectively the surface by transpiration. The char also helps block convection heating. In addition, in high heat flux environments, the char will sublime. Charring ablators do provide multiple levels of protection. However, they cannot be reused, and they tend to have high drag coefficients. More important, when the ash deteriorates or wears away and comes off, it tends to erode other areas of the space vehicle, even adversely affecting the tiles. Further, when a vehicle exceeds hypersonic speeds in the atmosphere, the surrounding air ionizes and creates a plasma sheath that tends to block telemetry and communication. Materials that ablate exacerbate this phenomenon.
In the light of the foregoing discussion it is to be appreciated that thermal protection materials are, and always have been, among the most crucial and critical technologies involved in access to space. There are not that many areas where materials technology is so visibly enabling. As evidence of the fact that metals, ceramics and ablative materials are not entirely satisfactory, an estimated forty thousand hours of maintenance is spent between shuttle flights on refurbishing and replacing components. There is, then, a clear need for a non-ablative thermal protection system for space transportation systems. An object of this invention is to provide such a TPS.